Today I Found Out has created a narrated video that explains why superheroes wear underwear on the outside of their costumes. The script for this video is an excerpt from the book The Wise Book of Whys by Today I Found Out.
While there have been many fantastical proposed origins of this seemingly odd modish style amongst comic artists — my favorite of which being that most superheroes lost their parents at an early age, so they had no one to tell them underwear goes UNDER your clothes — the true origin is pretty simple. According to Julius Schwartz (famed editor of DC Comics from 1944-1986 who edited the most famous of all external-underwear superheroes, Superman), this was simply modeled after the garb of aerial circus performers and wrestlers of the era in which the first superheroes proudly donned their underpants over their tights. (Read more).